Embodiments of the present invention pertain generally to apparatus and methods for vaporizing and mixing a vaporized liquid with a carrier gas. Embodiments of the invention are particularly suited for supplying vaporized reactants to the reaction chamber of a chemical vapor deposition system, for example, in semiconductor device manufacturing equipment.
Chemical vapor deposition (CVD) processes are widely used in the deposition of thin films used in semiconductor devices and integrated circuits. Such processes involve deposition resulting from a reaction of chemical vapors homogeneously or heterogeneously on a substrate. The reaction rate is controlled by one or more parameters, such as temperature, pressure and reactant gas flow rates. The use of low vapor pressure liquids as precursors for such processes has several advantages and has become more common.
Prior CVD processes involve transport of low vapor pressure liquid using a bubbler or boiler. In these processes, a carrier gas saturates the liquid and transports the vapor. Various liquid reactants and precursors are used in CVD applications by delivering the liquid reactants in a carrier gas. In liquid reactant CVD systems, the carrier gas is typically bubbled at a controlled rate through a container of the liquid reactant so as to saturate the carrier gas with liquid reactant and the saturated carrier is then transported to the reaction chamber.
Attempts have been made to deliver solid reactants to a CVD reaction chamber, but with much less success. The delivery of solid precursors in CVD processing is carried out using the sublimator/bubbler method in which the precursor is usually placed in a sublimator/bubbler reservoir which is then heated to the sublimation temperature of the precursor to transform it into a gaseous compound which is transported into the CVD reactor with a carrier gas such as hydrogen, helium, argon, or nitrogen. However, this procedure has been unsuccessful in reliably and reproducibly delivering solid precursor to the reaction chamber for a number of reasons. The major problems with the technique are centered on the inability to consistently vaporize a solid at a controlled rate such that a reproducible flow of vaporized solid precursor can be delivered to the process chamber. Also, it is difficult to ensure complete saturation of the fast flowing carrier gas stream because of a limited amount of exposed surface area of the solid precursor in the vaporizer system and lack of uniform temperature to provide maximum sublimation. Although solid precursor sublimator/bubbler systems and liquid precursor bubbler systems are both used for the delivery of CVD reactants, each of these systems has different problems and considerations. Therefore, a system or apparatus used for a solid sublimator/bubbler will not necessarily work for a liquid precursor bubbler apparatus.
Prior art apparatus for delivering the vapor formed by bubbling a carrier gas through a liquid precursor are shown in FIGS. 1A and 1B. FIG. 1A shows a prior art vaporizer apparatus 10 including an ampoule or vessel 12 containing a liquid precursor material 11. Gas inlet tube 14 is connected to a source of carrier gas 30. Gas inlet tube 14 extends beneath the level of liquid 11. Pressurized delivery of carrier gas 30 provides a mixture 32 of vaporized liquid precursor and carrier gas which then exits the vessel 12 through outlet conduit 16, which is connected to a CVD system (not shown).
The diffuser material 20 is typically a porous, sintered metal, and improves the bubbling efficiency of the vaporizer apparatus 10. The vaporizers shown in FIGS. 1A and 1B deliver vapor from material in a liquid state to a process chamber by heating the liquid material in a container and introducing the carrier gas at a controlled rate into the liquid material near the bottom of the container. The carrier gas then becomes saturated with vapor from the liquid material as the carrier gas bubbles to the top of the container. The saturated carrier gas is then transported to the process chamber, for example, a CVD apparatus used in semiconductor manufacture.
In the apparatus shown in FIGS. 1A and 1B, bubbles of carrier gas produce undesirable small droplets of the liquid precursor, which may be referred to as microdroplets. The microdroplets are carried together with the mixture of carrier gas and precursor vapor into the outlet tube and to the process chamber. Such microdroplets can cause defects in the finished products.
A need therefore remains for liquid vaporizer methods and apparatus which can vaporize liquid at flow rates sufficient for CVD processes and that reduce or prevent the delivery of small droplets of liquid to the process chamber.